1. Field of the Invention
This invention relates to the use of ammonia (NH.sub.3) for the desulfurization [removal of sulfur dioxide (SO.sub.2)] of gases resulting from the combustion of sulfur containing hydrocarbons which are commonly called flue gases. The product of the reaction of NH.sub.3 with SO.sub.2 predominantly is ammonium sulfate (NH.sub.4).sub.2 SO.sub.4 which is widely used as a source of nitrogen in materials such as fertilizers.
2. Description of the Prior Art
Processes for the desulfurization of gases containing SO.sub.2 currently being evaluated to achieve the degree of desulfurization of flue gases proposed recently by the President of the United States are based on the use of calcium oxides and combinations of calcium oxide and oxides of the alkaline earth elements. These calcium oxide based and calcium oxide, alkaline earth oxides mixtures cannot be regenerated and must be discarded into landfills. As a result of more stringent enforcement of the regulations regarding landfills by the Environmental Protection Agency, (EPA) the number of landfills in the United States has decreased from 14,000 to 6000 in the last several years. It is estimated that there will be a further decrease of 33% in the number of landfills in the next several years. As a result the price of placing a ton of waste material into a landfill has increased from about $6 when 14000 landfills were in operation to four or five times that amount at present. When the number of landfills has been further reduced, the price of placing a ton of material in a landfill will increase further. It is estimated by the Wall Street Journal that 15 states will have no landfills available in 10 years.
If the recommendations of the President with regard to reduction of the components of acid rain are approved by the Congress, the large increase in partially sulfated calcium oxide sorbents resulting from SO.sub.2 removal will occur at approximately the same time as the acute shortage of landfill sites. Therefore, there is a need for a method for reducing the SO.sub.2 emissions from power plants that is based either on the use of regenerable sorbents or the use of a process that creates a sulfate material that is an item of commerce. The use of NH.sub.3 for SO.sub.2 removal from flue gases meets these requirements because they would result in the formation of (NH.sub.4).sub.2 SO.sub.4 which is one of the most widely used chemicals known.
There are no research projects being funded in the current phase of the Clean Coal Technology Demonstration Program of the Department of Energy related to the use of NH.sub.3 for the desulfurization of flue gases.
NH.sub.3 is used in combination with catalysts for the Selective Catalytic Reduction (SCR) of nitrogen oxides (NO.sub.x) However in Request For Proposal (RFP) by the Department of Energy (DOE) [No. DE-RP22-89PC89801] it was stated: "Depending on the lifetime of an SCR catalyst, annualized control costs (for SCR reduction of NO.sub.x with NH.sub.3) are likely to be thousands of dollars per ton of NO.sub.x reduced from a high sulfur coal." The RFP further states: "Commercially available combustion modification techniques (e.g., certain low-NO.sub.x burners) and flue gas treatment processes (e.g. selective catalytic reduction) and selective noncatalytic reduction processes will not qualify" (as a technique applicable to this proposal).
The statements on the inapplicability of SCR removal of NO.sub.x with NH.sub.3 is based on a report from the Electric Power Research Institute (EPRI) EPRI CS-3606, "Selective Catalytic Reduction for Coal-Fired Power Plants: Feasibility and Economics", Oct. 1984. This work documented the research effort by EPRI on the catalytic reduction of NO.sub.x with NH.sub.3. The operating range of the catalyst was specified by the manufacturer to be 580.degree. F. to 750.degree. F. This required that the catalyst be placed in operation between the economizer and air preheaters of the boiler. The investigators showed that the catalyst did result in the reduction of NO.sub.x to nitrogen (N.sub.2). The process was less than satisfactory because of the incomplete utilization of the NH.sub.3 used. Furthermore, the investigators concluded that there was a conversion of 1.4% of the SO.sub.2 by catalytic oxidation to SO.sub.3. The unreacted NH.sub.3 and SO.sub.3 may have reacted to the fly ash. EPRI has reported the formation of compounds such as: NH.sub.4 Al(SO.sub.4).sub.2, NH.sub.4 Al(SO.sub.4).sub.2 .times.12 H.sub.2 O which account for over 42% of the deposits found in the air preheaters which were designed to have an exit temperature of 331.degree. F. (161.1.degree. C.). (Al.sub.2 O.sub.3 constituted 25% of ash in the coal used in this trial.) These precipitates increased the pressure drop in the air preheaters to a level that interfercd with the efficient operation of the boiler.
Applicants have determined by thermodynamic calculations that SO.sub.2 may be removed with NH.sub.3 without the utilization of the catalyst for the conversion of SO.sub.2 to SO.sub.3 . However, use of a catalyst to convert SO.sub.2 to SO.sub.3 may be accelerated by the use of a catalyst.